The aim of a color gamut mapping is to redistribute the source colors belonging to a source color gamut (for example the extended color gamut of a film) into a target color gamut (for example the color gamut of a standard television monitor). As the shape and boundaries of a target color gamut are generally different from those of the source color gamut, at least some of the target colors that are obtained after such a mapping are different from their corresponding source colors.
An application area of color gamut mapping is notably video content production and post-production. For example, an original version of a video content may need to be converted into different versions adapted for different types of reproduction or transmission: for example, a specific version for cinema, another for television, and a third one for internet. These different versions can be prepared by manual color correction or/and by application of gamut and tone mapping algorithms.
Among the requirements for color gamut mapping are notably:                preservation of color neighborhood and order, absence of color banding and false contours, in order, notably, to prevent from incoherent reproduction of grey and color ramps;        continuity of color and absence of visible quantization or clipping errors, in order, notably, to prevent from banding and false contours;        separate control for lightness, hue and saturation for keeping the full artistic control on how colors are modified, and for allowing the formulation of a higher, semantic level of artistic intents.        
In order to define a color gamut mapping, a gamut boundary description (GBD) of the source color gamut and of the target color gamut is generally required. Such a GBD of a color gamut defines the boundary surface of this color gamut in a color space. GBDs comprise generally explicit, generic 3D representations such as triangle meshes or volume models. For instance, a GBD of a color gamut can be based on a mesh of triangles, each triangle being defined by its three vertices in the color space of this GBD, these vertices corresponding to colors located on the boundary of the color gamut.
As illustrated by dotted lines on FIG. 1 in a RGB color space, in case of a color gamut of a trichromatic display or a trichromatic camera, cusp lines usually correspond to singular lines (“edges”) linking each primary color of this display or camera with a secondary color having this primary color as a component, namely a singular line linking: red with yellow, red with magenta, green with yellow, green with cyan, blue with cyan and blue with magenta. The “cusp line” of a color gamut is a line joining cusp colors of this gamut. When the color gamut is represented in a color space having an axis for chroma such as Lab or JCh color space, a cusp color is a color of maximum Chroma (i.e. maximum saturation) in this color gamut and in a plane defined by a constant hue in this color space. In Lab color space for example, chroma is defined to be the square root of the sum of the squares of a and b, respectively. A plane defined by a constant hue is generally named “constant hue leaf”. More generally, cusp colors correspond to singular points (“vertices”) or singular lines (“edges”) on the boundary surface that limits a color gamut. The cusp line of a color gamut can be generally modeled as a line joining different cups colors that forms a closed polygon on the gamut boundary of this color gamut.
As illustrated by solid lines on FIG. 1, “rims” of a color gamut correspond to the high-lightness ridges of this color gamut linking the white point of this color gamut to the secondary colors and to the low-lightness ridges linking the black point of this color gamut to the primary colors. For example, a yellow rim of a color gamut starts at the white point and ends at the yellow secondary color. The colors on this yellow rim include white, yellowish whites, pales yellows, saturated yellows and finally the yellow secondary color itself.
A cusp line or a rim of a color gamut include generally singular points that correspond generally to non-continues curvature of the gamut boundary of the color gamut.
On FIG. 1, cusp lines (dotted lines) and rims (solid lines) of the color gamut are by definition straight lines, because these lines are represented in the RGB color space defined by the device having the primary and secondary colors. The same lines are generally not straight when represented for instance in a Lab color space.
When trying to define a method of color gamut mapping (or algorithm: “GMA”) source colors inside a source color gamut (having its own source cusp line and source rims) into target colors such they are located inside a target color gamut (having its own target cusp line and target rims), notably in order to take advantage of the whole range of colors in the target color gamut, it is known to define the GMA according different conditions among which the following cusp mapping condition: any source cusp color should be mapped into a target cusp color. More generally, a GMA can be defined in reference to pairs of source cusp colors and target cusp colors. Such color mapping methods are known as “cusp color gamut mapping”. The general diagram shown on FIG. 4 illustrates such a cusp color gamut mapping where the color mapping is performed in a mapping color space different from that of a source device and from that of a target—or destination—device, requiring the use of a source display model to transform RGB color coordinates representing source colors in the RGB color space of this source device into a representation of these colors in the mapping color space, and the use of an inverse target display model to transform representation of mapped colors in the mapping color space into R′G′B′ color coordinates representing mapped colors in the R′G′B′ color space of this destination device.
US2007/236761 discloses a mapping method using cusp colors of a color gamut. In the disclosed method, a simplified cusp representation is used where cusp colors that are different from primary and secondary colors of the color gamut are interpolated from primary and secondary colors. In the disclosed method, a color ([0104] “point A”) is mapped ([0104] ‘chroma dependent lightness mapping”) to a mapped color ([0104] “point B”). The mapped color has a lightness that is closer to the lightness of a cusp point of the constant-hue leaf of the color to map ([0104] “lightness compression toward primary cusp point”). This cusp point is that of a target gamut and is identical to the cusp point of a source gamut of the same hue leaf (FIG. 12b: “both cusp points”) after a cusp point mapping in this constant-hue leaf ([0059] “the source primary cusp point is mapped to the destination primary cusp point”) and after mapping of black and white points of the source gamut to the black and white points, respectively, of the target gamut (FIG. 11: “lightness rescaling”). The lightness mapping then depends on the unique black point, the unique white point and the unique cusp point in the constant-hue leaf in which the lightness mapping is performed.
A drawback of the color mapping method disclosed in US2007/0236761 is that it is based on a unique cusp point and not at least on two different cusp points, those of source and target color gamuts.
US2005/248784 discloses a color gamut mapping method called shear mapping that maps in a constant hue leaf the cusp of the source gamut to the cusp of the target gamut. However, after the shear mapping, colors different from the cusp colors may still lie outside of the target (or “destination”) color gamut. For such a situation, US2005/248784 discloses an additional step that further map colors 610 that lie outside the target color gamut to the closest colors 610′ of the target color gamut, see FIG. 10 of US2005/248784 reproduced as FIG. 2 below. The document EP2375719 discloses also such an additional mapping step.
A drawback of the gamut mapping method disclosed in US2005/248784 is that this additional clipping step destroys image details and color neighborhood.